1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus and a charging voltage control method. More particularly, the present invention relates to an electrophotographic image-forming apparatus and a charging voltage control method capable of preventing print quality deterioration due to uneven surface potentials of a photosensitive medium caused by resistance value changes resulting from aging or poor contacts of a charging roller or a transfer roller.
2. Description of the Related Art
In general, the electrophotographic image-forming apparatus is employed in image-forming devices such as laser beam printers, LED print head (LPH) printers, copiers and facsimile machines. Such an electrophotographic image forming apparatus performs printing jobs through the process of charging, exposing, developing, transferring and fusing.
FIG. 1 is a cross-section view schematically showing a conventional electrophotographic image-forming apparatus. Referring to FIG. 1, an electrophotographic image-forming apparatus has a photosensitive drum 10, a charging roller 20, a laser scanning unit (LSU) 30, a developing roller 40, a transfer roller 50, a high voltage power supply (HVPS) 60, and a control unit 70.
During printing operations the HVPS 60 applies predetermined voltages to the charging roller 20, developing roller 40, and transfer roller 50 according to the controls of the control unit 70. The charging roller 20 uniformly charges the surface of the photosensitive drum 10 with the charging voltage applied from the HVPS 60. The LSU 30 scans light on the photosensitive drum 10 corresponding to image data input from the control unit 70. Accordingly, an electrostatic latent image is formed on the surface of the photosensitive drum 10.
Thereafter, the electrostatic latent image formed on the surface of the photosensitive drum 10 turns into a toner image with toner supplied by the developing roller 40. The transfer roller 50 driven by the transfer voltage applied from the HVPS 60 transfers onto a sheet of recording paper the toner image formed on the photosensitive drum 10. The toner image transferred onto the sheet is fixed on the sheet of printing paper by applying high heat and pressure with a fusing device (not shown), and the sheet is discharged to the outside along the discharging direction and printing is completed.
The conventional electrophotographic image-forming apparatus brings concentration deviation out on the image recorded on the recording paper, which results in a poor print image when the surface potential formed on the photosensitive drum 10 becomes uneven while the print job is performed. Accordingly, it is beneficial to apply a constant charging voltage in order to maintain a uniform surface potential of the photosensitive drum 10. However, even though the constant charging voltage is applied to the charging roller 20, the charging potential of the photosensitive drum 10 may vary since resistance values of the respective rollers are changed due to ambient environment changes, for example, temperature and humidity changes. Therefore, the conventional electrophotographic image-forming apparatus determines the charging voltage to be applied to the charging roller 20 in consideration of such resistance value changes due to the environment changes.
For example, the conventional electrophotographic image-forming apparatus determines the charging voltage to be applied to the charging roller 20 based on the resistance value of the transfer roller 50. That is, the conventional electrophotographic image-forming apparatus detects the resistance values of the transfer roller 50 according to the ambient temperature and humidity changes, and varies the charging voltage to be applied to the charging roller 20 based on the detected resistance value, to compensate for print quality.
However, the resistance value of the transfer roller 50 may also vary due to mechanical defects such as aging or poor contacts of the transfer roller 50, in addition to the ambient environment changes. FIG. 2 shows the relationship between the aging and resistance values of the transfer roller 50 where the resistance value of the transfer roller 50 increases as the transfer roller 50 is getting older. As the transfer roller is used, the resistance value of the transfer roller 50 increases and overvoltage is applied to the charging roller 20, causing printed images that are blurred or image quality deterioration by the occurrence of a pinhole.
For another example, the electrophotographic image-forming apparatus determines a charging voltage to be applied to the charging roller 20 based on the resistance value of the charging roller 20. That is, the electrophotographic image-forming apparatus detects a resistance value of the charging roller 20, varies the charging voltage to be applied to the charging roller 20 based on the detected resistance value, and compensates for the variation of the surface potential of the photosensitive drum 10 according to printing environments. Even in this case, the measured resistance value of the charging roller 20 can be higher than the resistance value in actual environments due to mechanical defects such as poor contacts of the charging roller 20. If the resistance value of the charging roller 20 increases, the amount of toner applied on an electrostatic latent image is reduced, causing printed image deterioration.
As above, where a charging voltage to be applied to the charging roller 20 is determined by considering only one of the resistance values of the transfer roller 50 and the resistance value of the charging roller 20, an overvoltage is applied to the charging roller 20 because the resistance value of the transfer roller 50 or the charging roller 20 increases due to its mechanical defects such as aging or poor contacts. When an overvoltage is applied to the charging roller 20, a problem occurs as print images become blurred with lower image concentration.